1. Field of the Invention
The present invention relates to a double-sided image forming apparatus and method in which document images are read, toner images to be printed on the front and back sides of a copy sheet are formed from the read images, and the toner images are transferred and fixed to the front and back sides of the copy sheet so that double-sided image formation is carried out.
2. Description of the Related Art
In an image forming apparatus that performs double-sided image formation, the first page image formed on the photoconductor drum is transferred and fixed to the copy sheet, and it is once contained in the double-sided reversal feeding device. Synchronized with the second page image being formed on the photoconductor drum again, the copy sheet is supplied from the double-sided reversal feeding device, and the second-page image is transferred and fixed to the copy sheet, so that double-sided image formation is carried out.
In the above-mentioned technique, the conveyance of the copy sheet to the double-sided reversal device and the twice fixing of the toner images at the fixing device are needed. It has been pointed out the problem on the reliability of copy-sheet conveyance and the occurrence of paper jam.
In order to cope with this problem, a double-sided image forming apparatus has been developed in which the toner image supporting medium is provided as a middle transfer medium for transferring temporarily the toner image on the photoconductor drum to the toner image supporting medium, and the toner image forming device. This toner image forming device includes the document reader, the image memory which accumulates the read document image, the photoconductor drum, the charging device, the writing unit, the developing device, etc.
In the above-mentioned image forming apparatus, the document is automatically read by the document reader, and the image data of the read document is accumulated in the image memory. By reading the image data of the second page from the image memory, the image formation on the photoconductor drum is performed. The image on the photoconductor drum is transferred to the toner image supporting medium. After this, the image data of the first page is read from the image memory, and the image formation on the photoconductor drum is performed. And the synchronization of the paper feeding, the toner image supporting medium, and the photoconductor drum is taken, the images of the first page and the second page are transferred to both sides of the copy sheet, respectively, so that double-sided printing is carried out.
Moreover, when the first-page document is manually placed on the contact glass (document stand) of the digital copier, the CPU determines whether the document size is less than A4 size according to an output signal of the document size detector. When it is A4 size or less, the two-sheet mode is chosen in the above-mentioned image forming apparatus. The front-side image of the first sheet is stored in the image memory, the front-side image is formed with the toner, and the toner image is transferred to the toner image supporting medium. Next, the front-side image of the second sheet is formed with the toner, and the toner image is transferred to the toner image supporting medium.
Namely, the toner images for the front-side images of the first and second sheets are formed on the toner image supporting medium in the two-sheet mode. Next, the toner image supporting medium is separated from the photoconductor drum, the back-side image of the second sheet is read, and the image data is stored in the image memory. And by reading the image data from the image memory, the back-side images of the two sheets are formed on the photoconductor drum, and the paper feeding of the copy sheets is started. And the back-side images are transferred from the photoconductor drum to the copy sheets in a consecutive manner.
Next, the front-side images are transferred from the toner image supporting medium to the copy sheets, and the copy sheets with the double-sided printed images are passed through the fixing device, and the paper ejection is performed. See Japanese Laid-Open Patent Application No. 10-186737 for the above-described technique.
However, in the above-mentioned image forming apparatus, the sequence of reading of respective document images and the sequence of developing/transferring of the toner images are not taken into consideration, and a relatively long time is needed for double-sided printing of a first copy sheet among a plurality of copy sheets. In other words, the first copy time is relatively long. This is not convenient for the user of the image forming apparatus, and the user feels the waiting in a long time to obtain the first copy sheet. For this reason, the user-friendliness of the image forming apparatus deteriorates.
Moreover, when a paper jam occurs within the first copy time, the double-sided printing speed of the image forming apparatus as a whole is significantly lowered due to the paper jam.